Toward the understanding of annealing effects on (GaIn)2O3 films
Introduction
Wide-bandgap materials offer the possibility of fabricating short wavelength optical emission, deep-ultraviolet transparent electrode, deep-ultraviolet transparent photodetector and high power density electronic devices [1]. (GaIn)2O3 alloys are desirable for these applications as they enable a tunable wide bandgap across the composition range, which spans 3.6 eV in In2O3 to 5.1 eV in Ga2O3 [2], [3], [4]. The structure of (GaIn)2O3 alloys can be that of either β-Ga2O3 or cubic In2O3, depending on the Ga/In ratio in it [3], [5], [6], [7]. Unfortunately, the difference of the thermally stable crystal shapes of β-Ga2O3 (monoclinic) and In2O3 (cubic) causes phase separation (the simultaneously existing of both monoclinic and cubic phases) easily, which greatly degraded the crystalline quality of (GaIn)2O3 films [2]. Suzuki et al. [8] have tried to solve it by the fabrication of rhombohedral corundum-structured α-(GaIn)2O3 films. However phase separation still appeared when indium content was between 0.08 and 0.67. We also found that although the indium content of (GaIn)2O3 films measured by energy dispersive X-ray spectroscopy can be controlled by the element composition in the targets, and the bandgap of (GaIn)2O3 films can be tailored between 3.8 eV and 5.1 eV, the phase separation was observed for the films with nominal indium content (weight ratio of In/(Ga + In) in target) ranging from 0.2 to 0.5 (indium content in (GaIn)2O3 films ranging from 0.18 to 0.33) [4]. Fortunately, further research found that by annealing the film (nominal indium content of 0.3) deposited at room temperature in air ambient, high quality β-(GaIn)2O3 film (with indium content of 0.2 in the film) without phase separation can be obtained [9].
However, the mechanism of the annealing effect is still obscure since the ambient air is a mixture of gases mainly composed of N2 and O2. Many researchers found that inert gas annealing is effective for crystallizing the amorphous Ga2O3 films [10], [11], [12]. Huang et al. annealed the epilayer at 800 °C under different ambient to improve the crystallinity and found that the X-ray diffraction of β-Ga2O3 annealed in a nitrogen environment for 15 min has the strongest peak while annealing in an oxygen environment for 30 min shows diminished performance due to the oxygen content reaching a saturation state during annealing. In air ambient annealing which used both oxygen and nitrogen, the observed crystalline properties were better than those obtained from purely oxygen annealing [13]. Up to now, no reports have systematically studied the annealing effect of (GaIn)2O3 films. Thus, in the present work, in order to understand the annealing effect, we carry out further research on the (GaIn)2O3 film with same nominal indium content of 0.3 by investigating the annealing gas ambient and temperature influences. In addition, high oriented (GaIn)2O3 films with a wide nominal indium content (from 0.2 to 0.7) were obtained by post-annealing at the optimized conditions.
Section snippets
Experiments
The (GaIn)2O3 films for annealing were prepared by pulsed laser deposition on (0001) sapphire substrates at room temperature (RT). Details of the deposition parameters have been described elsewhere [14] and listed in Table 1. Post-annealing was carried out with a quartz tube furnace with samples on a quartz boat placed into the center of it for one hour. By changing the ambient (N2, vacuum, Ar, O2) and temperature (700–1000 °C), the annealing parameters were optimized. The annealing effect on
Influence of annealing gas ambient
X-ray diffraction patterns of (GaIn)2O3 film with nominal indium content of 0.3 annealed at 800 °C using different gas ambient are shown in Fig. 1. Films annealed in N2, vacuum and Ar are of amorphous structure due to the absence of a distinguishable diffraction peak. Film only annealed at oxygen shows three diffraction peaks which can be assigned as (− 201) and its higher orders of monoclinic (GaIn)2O3 according to the JCPDS card (No: 43-1012). This reveals that the film becomes textured with (−
Conclusion
(GaIn)2O3 films with nominal indium content of 0.3 as-deposited in room temperature have been annealed in different gas ambient (N2, vacuum, Ar, O2) and temperatures (700–1000 °C) at first. It is found that gas ambient and temperature have important influence on crystal quality of annealed (GaIn)2O3 films. Only oxygen ambient can crystallize (GaIn)2O3 film and film annealed in 800 °C appears best crystal quality. By annealing (GaIn)2O3 films with different nominal indium content varying from 0.2
Acknowledgments
This work was partially supported by the Partnership Project for Fundamental Technology Researches of Ministry of Education, Culture, Sports, Science, and Technology, Japan. We are grateful to Doctor Y. Cui for help on data fitting and discussion.
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